40. Originally, "progressives" were Republicans and northern Democrats around 1900

They started out supporting some particular issues like women's suffrage, temperance, child labor laws and labor reform, etc. They also believed in what was called "corporatism", that is to say, enlisting businesses and associations in effecting social change. Progressives include Teddy Roosevelt, Woodrow Wilson, and, oddly enough, the last Progressive (in that sense) President was Herbert Hoover (who was not the President a lot of us seem to think he was). Progressivism also had some racial problems; there was always a tinge of anti-semitism (think Henry Ford), and the 1920s incarnation of the KKK was "progressive" (this is the Klan that was big in Indiana, for instance, and mostly agitated against alcohol and southern European immigrants). FDR ran as a Bourbon Democrat as opposed to the Progressive Democrats like, wait for it, Strom Thurmond.

The new sense of "progressive" was coined by "new Democrats" who didn't like the label "liberal". I associate its popularization with James Carville's book "We're Right and They're Wrong: A Handbook for Spirited Progressives", but it probably had a lot of use before that in the 1980's.

I'm wary of the term, personally.

"Liberal" is a problematic word, since for most of the world it means largely the opposite of what it does in the US. In the US, "liberal" means statist economic policies and individualist social policies. Our social policies don't really have analogues in other advanced countries, so they just hear us call US Democrats "liberal" and scratch their heads (the UK press in 2008 were looking at the collapsing economy and writing about "the failures of liberal economics", which in turn caused head-scratching on this side of the pond).

On a separate note, conservative political philosopher Noah Millman suggests "progressive vs. conservative" is one spectrum of political thought, and "left vs. right" is another: progressives like to make new institutions and get rid of old ones; conservatives like to adapt existing institutions (there are, in that usage, progressives and conservatives on both the right and the left). I think this definition is very useful, but it hasn't really caught on.